This research deals with behavioral sex differences in the first year of life. Behaviors have been chosen for which sex differences have been shown to be reasonably reliable, and which have proved predictive of behavior at later ages. The research seeks to determine the extent to which biochemical factors underly these behavioral sex differences. The relationship between sex hormones and behavior will be studied in two groups of human infants; Group 1. Studied longitudinally, at day 2, at 3 weeks and at 6 months. Group 2. Studied longitudinally, at age 3 months and at 13 months. For all infants studied, blood samples will be taken at birth and assayed for testosterone, androstenedione, and estrogen. Behaviors to be studied during infancy include: neonatal tactile sensitivity and motor coordination; quieting; sleep-wake cycle; responsiveness and attention to visual and auditory stimulation; barrier behavior, and latency to resume normal activity after mild stress. Maternal behavior and its relation to both the infants biochemical scores and their behavior will also be studied.